There are many devices which consume hydrogen and produce power. For example, proton exchange membrane fuel cells are being developed currently as power sources for a variety of military, transportation, and electronic device applications. Such fuel cells require that hydrogen, sufficient to generate the required power output, be available when required. Meeting this requirement calls for a high density and energy efficient hydrogen storage technology.
Many such technologies have been proposed and studied including devices that store hydrogen: as compressed hydrogen gas; as cryogenic liquid hydrogen; as hydrogen molecules adsorbed on high surface area supports; as hydrogen atoms at low density in metallically bonded solid transition metal hydrides; as hydrogen atoms at high density in ionically bonded solid light metal hydrides; and as hydrogen atoms at high density in polar covalently bonded solid complex hydrides. Each of these approaches has limitations.
Those approaches which rely on chemical bonding to store and release hydrogen are attractive, but input of energy, conventionally as heat, is required for hydrogen release. In transition metal hydrides, hydrogen may be released at moderate temperatures because the hydrogen and transition metal are relatively weakly bonded with metallic bonds. But transition metal atoms have atomic weights of greater than approximately 50 atomic mass units and store, at most, approximately two hydrogen atoms per transition metal atom. Thus, the gravimetric storage density of transition metal hydrides is less than 4 weight percent hydrogen, which is too low for many applications.
Light metal atom hydrides can have high hydrogen densities, up to approximately 12 weight percent hydrogen. However, the ionic chemical bonds between the metal and the hydrogen in these hydrides are very strong and, therefore, high temperatures, beginning at about 280° C. and ranging up to 900° C. and greater are needed to release the hydrogen. These temperatures are impractical for many applications.
Hydrogen stored in polar covalently bonded light metal complex hydrides can have storage densities up to 18 weight percent hydrogen. Like light metal hydrides, these compounds are generally very strongly bound and therefore, again, high temperatures are required to release the hydrogen.
There is therefore need for improved methods of chemically storing and releasing hydrogen.